Field of the Invention
The invention relates to a method and a device for fixing toner images with a gas to which the toner images are exposed
For the purpose of fixing toner images in electrophotography and related processes, a fundamental distinction is drawn between contact and noncontact methods. The first group includes fixing with hot rolls or belts and pressure fixing, the latter, if necessary, under the influence of elevated temperature. The second group includes, for example, radiation methods using lamps which operate continuously or in a pulsed manner, or the physicochemical methods of fixing by solvent evaporation.
Among these methods, hot-roll fixing has achieved wide popularity to date, although it is accompanied by some disadvantages, which have to be paid for elsewhere through increased outlay or through losses in print quality. A great problem with which hot-roll fixing is beset, is so-called "hot offset", in which molten toner remains adhering to the roll surface and is deposited onto the substrate during subsequent revolutions. A characteristic "ghost image" is produced.
In order to counter this defect, the roll materials which are selected have a low surface energy (such as PTFE or silicone rubber, for example), and in addition a low-viscosity release agent, generally silicone oil, is applied to the surface thereof. These measures lead both to making the unit more expensive and also to the undesired application of silicone oil to the print, and thus to uncontrollable gloss. Likewise, in order to prevent hot offset, inconvenient requirements are placed upon the toner that is used; the mechanical moduli of the toner (visco-elastic properties) must be set so that an adequate elastic component counteracts the hot offset. This impairs the ability of the toner to flow during fixing, extending as far as suppressing the adequate coalescence of toner droplets. The impaired ability to flow also has a disadvantageous effect upon the process of toner production, because excessively elastic materials present difficulties during the grinding process. Furthermore, in order to suppress hot offset, internal release agents are added to the toner, which in turn complicate the production of toner and make the toner more expensive.
The second group of fixing methods, the noncontact methods, do not have the problems of hot offset. The toners can therefore be produced as "capable of ideal flow", no silicone oil and no internal release agents being needed. The disadvantages of the noncontact method relate to the controlled introduction of the necessary heat into the toner layer. In the case of all radiation systems, the thermal efficiency, at least at relatively high fixing speeds, is lower than in the case of roll fixing.
Continuously operating radiation systems have, in substance, a safety problem, which has to be overcome at the expense of outlay on machine construction. This is because, if the paper transport is interrupted, for example, by a paper jam, the risk of ignition of the paper is high. For this reason, these systems are normally used in web-fed presses wherein the paper transport can be monitored relatively easily, but not in sheet-fed presses.
Radiation methods operating in a pulsed manner, so-called flash-fusing systems, often produce local overheating of the toner layer, which leads to thermal degradation of the polymers and therefore to the emission of unhealthy and unacceptably smelly gases. In addition, it has been reported that, as a result of the rapid heating-up of the toner layer, the latter tends toward microexplosions, the traces of which prevent uniform area filling.
The method of fixing with solvent vapor operates in accordance with the principle that the toner layer on the substrate is caused to swell by the vapors. As a result, a liquid ink film is produced on the substrate, is basically able to behave like a liquid printing ink and should deliver potentially high image quality. Following the fixing, the solvent is removed from the substrate. The disadvantages of the method are obvious: operating with organic solvents in a printing press is undesirable from the aspects of environmental protection and health and safety at work. In addition, heretofore known systems are also still based on halogenated solvents (CFC), the use of which is being considered less and less.
All heretofore known methods, which operate with temperatures significantly above 100.degree. C. (typical fixing temperatures are around 170.degree. C.) also damage the paper, that is the most important printing substrate, in that they drive out the water contained therein, leading to deformation.